Lonely, fail to go back into romantic meeting (8)
I believe the answer is:
desolate
'lonely' is the definition.
(I know that lonely can be written as desolate)
'fail to go back into romantic meeting' is the wordplay.
'fail' becomes 'lose' (I've seen this in other clues).
'to go back' shows that the letters should be reversed in order.
'into' indicates putting letters inside.
'romantic meeting' becomes 'date' (eg a dinner date).
'lose' in reverse letter order is 'esol'.
'esol' put inside 'date' is 'DESOLATE'.
(Other definitions for desolate that I've seen before include "Barren" , "Bleak, dismal; wretched" , "upset" , "'Bleak, bare (8)'" , "Wretched, unhappy" .)